Chiral cruciferous phytoalexins: Preparation, absolute configuration, and biological activity

Synthesized by an efficient one-pot spirocyclization method, two chiral cruciferous phytoalexins, 1-methoxyspirobrassinin (2) and 1-methoxyspirobrassinol methyl ether (4a), were prepared through optical resolution using the chiral HPLC method of corresponding racemates. The absolute configuration of natural (+)-2 was elucidated as R by using the direct comparison of ECD and VCD spectra with those of known (S)-(−)-spirobrassinin (1). Another chiral phytoalexin, (−)-4a, had its absolute configuration 2R,3R elucidated through the comparison of observed and calculated VCD. Interestingly, the absolute configurations of natural (S)-(−)-spirobrassinin (1) and (R)-(+)-1-methoxyspirobrassinin (2) were opposite of each other, even though their structures are almost similar, with the exception of an N-methoxy group. A significant difference in the antiproliferative activity between (2R,3R)-(−) and (2S,3S)-(+)-4a was observed.

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